Considering that you do not have ball joints on the rear axle that leads me to believe you are working on the front. No ZJ ever came with a limited slip front end so I would say that unless someone put one in later then you do not have one.

Look for the tag on the rear diff cover. If you have lsd, there will be a tag that says "use limited slip additive only" or something like that. If you dont have lsd, there will just be the normal gear ratio tag
If you're still not sure, just put in the additive anyway. Even if you dont have lsd, the additive wont hurt anything. But if you do have LSD and you dont put the additive, you can hurt the clutches

except your wallet though some brands of gear oil include the additive (IIRC valvoline synpower does and it's cheap)

when i changed my diff fluid (i have lsd) i bought the additive from the dealer for 15 bucks. if you dont know if you have lsd, thats still way cheaper than having the lsd rebuilt if you fry it by not putting the additive in.